In the Claus process, elemental sulfur is produced by reacting H.sub.2 S and SO.sub.2 in the presence of a catalyst. The Claus system uses a combustion chamber which, at 950.degree.-1350.degree. C., converts 50 to 70% of the sulfur contained in the feed gas into elemental sulfur. The sulfur is condensed by cooling the reaction gas to a temperature below the dew point of sulfur, after which the remaining gas is heated and further reacted over a catalyst. Normally, the gas passes through at least two such Claus catalyst stages.
The different stages of the process may be represented by the following equations: ##EQU1##
Below 500.degree. C., the symbol n has a value of approximately 8.
The final Claus exhaust gas still contains small amounts of H.sub.2 S, SO.sub.2, CS.sub.2, carbon oxysulfide, and elemental sulfur in the form of a vapor or mist. The exhaust gas is usually subjected to post-combustion to convert everything to SO.sub.2, which is then emitted into the atmosphere.
The sulfur emitted as SO.sub.2 into the atmosphere with the exhaust gas may amount to 2-6% of the sulfur contained in the feed gas in the form of H.sub.2 S. In view of air pollution and the loss of sulfur involved, further purification is imperative.
Claus aftertreatments have been developed. These are carried out after the last Claus stage or after the post-combustion. These aftertreatments are, however, complicated and expensive or inadequate.
One aftertreatment, carried out before post-combustion, seeks to achieve by catalytic conversion as complete a reaction as possible between H.sub.2 S and SO.sub.2. The reaction temperature is lowered to below the condensation point of sulfur, whereby the reaction equilibrium corresponding to equation II is shifted to form sulfur. A distinction is made between dry processes using alternating reactors in which the catalyst is intermittently charged with sulfur and discharged, and processes where H.sub.2 S and SO.sub.2 react in a high-boiling catalyst-containing liquid to form elemental sulfur which is drawn off continuously as a liquid product.
Unfortunately, in these processes any deviation from the optimum H.sub.2 S:SO.sub.2 ratio in the Claus exhaust gas results in a reduced sulfur yield. No appreciable conversion of sulfur compounds such as COS and CS.sub.2 occurs. Sulfur recovery efficiency of Claus using this form of aftertreatment is limited to 98-99%. Cyclic operation, with alternating reactors, requires at least two reactors and much valves and piping.
A second aftertreatment catalytically hydrogenates SO.sub.2 and S with H.sub.2 and CO while COS and CS.sub.2 are simultaneously hydrolyzed with H.sub.2 O into H.sub.2 S, which can be treated conventionally.
Hydrogenation/hydrolysis does not require a stoichiometric H.sub.2 S/SO.sub.2 ratio in the Claus exhaust gas. It almost completely converts COS and CS.sub.2 so that sulfur yields of more than 99.8% can eventually be obtained. This process requires high expenditure for elaborate apparatus and consumes a lot of energy. Recycle of H.sub.2 S reduces the Claus system capacity, while the production of waste water containing harmful constituents presents additional problems.
A third aftertreatment oxidizes all sulfur compounds into SO.sub.2 which is then further processed. These processes are downstream of the post-combustion and therefore independent of the mode in which the Claus system is run. There are also dry processes, where SO.sub.2 is adsorbed and returned to the Claus unit or processed to form sulfuric acid, and wet processes, where SO.sub.2 is removed by absorptive scrubbing and further processed. For complete oxidation of COS and CS.sub.2 into SO.sub.2, the energy requirements are high and following the after-combustion, very large exhaust gas flows have to be treated.
The equilibrium conversion of the Claus reaction (equation II) may be improved by condensing out part of the water in the gas. The gas is then reheated and charged to another Claus stage to form elemental sulfur. This produces waste water which is highly corrosive due to the formation of thiosulfuric acid, polythionic acids and sulfurous acid. Processing of such waste water is expensive. Unavoidable formation of deposits of elemental sulfur also occurs during H.sub.2 O condensation. Moreover, there is no conversion of COS and CS.sub.2 so the maximum recovery of sulfur is about 98%. As a result of these disadvantages, this process has not been used on a commercial scale.
Where the aftertreatment involves conversion of all sulfur compounds into hydrogen sulfide, it is also known to oxidize part of said hydrogen sulfide with air into SO.sub.2 or to convert part of the sulfur produced into sulfur dioxide and thereafter catalytically to convert the remaining hydrogen sulfide with sulfur dioxide at 125.degree.-150.degree. C. in fixed-bed reactors into sulfur. The sulfur loaded catalyst is regenerated by passing hot oxygen-free gases containing hydrogen sulfide through the catalyst. This avoids the disadvantages associated with the first type of aftertreatment, such as dependence on H.sub.2 S/SO.sub.2 ratio and COS/CS.sub.2 content in the Claus exhaust gas. Disadvantages of this process are the high capital cost and the higher H.sub.2 S +SO.sub.2 input concentration for the low-temperature reactor caused by the admixture of a separately produced flow of SO.sub.2. The maximum conversion overall efficiency obtainable with this process approaches 99%.
Direct catalytic oxidation of H.sub.2 S in gas mixtures with air or oxygen into elemental sulfur is also known. These processes are not sufficiently effective in the thermodynamically advantageous temperature range or the catalysts quickly lose activity. The conversion efficiency may be poor, with low sulfur concentrations because of the unfavorable reaction kinetics at low temperatures. Some processes lack selectivity for H.sub.2 S so that compounds, such as H.sub.2, CO and hydrocarbons, are oxidized as well. To avoid this, the H.sub.2 S oxidation may be done at temperatures below the dew point of sulfur, but then the catalyst becomes loaded with elemental sulfur and must periodically be regenerated. Many catalysts also lose activity due to adsorption of SO.sub.2 or sulfation. Proposed catalysts include bauxite, aluminosilicate zeolites, active carbon, oxides or sulfides of tungsten, vanadium, chromium, iron, cobalt, nickel, copper, molybdenum, silver, and manganese on supports, as well as alkali metal sulfides and combinations of alkali metal oxides with alkaline earth metal oxides.
DE-A-2,754,595 describes a process for cleaning the effluent gases of a Claus unit and uses a catalyst containing silver or 1 to 20% titanium dioxide when treating a gas containing carbon oxysulfide and/or carbon disulfide. In general terms, this publication is directed to the hydrolysis of COS and CS.sub.2 using a reducing atmosphere, page 12, lines 2-9, stating that it is preferred to have a hydrolysis step in the substantial absence of oxygen.
U.S. Pat. No. 4,414,962 describes the treatment of carbon derivatives of sulfur in the Claus process and recognizes that the best way to destroy COS and CS.sub.2 is by hydrolysis. That document notes that TiO.sub.2 is a good catalyst for such a reaction but emphasizes that high TiO.sub.2 concentrations are to be avoided.
EP-A-136,741 is concerned with improving TiO.sub.2 catalysts for use in the Claus process. Oxygen may be present, but it is stated at Page 20, lines 1-4 that oxygen causes no change to the yield.
There is a great need for a simple process which can be used downstream of conventional Claus units to clean up the Claus gas effluent. It would be especially beneficial if a process could be found which can satisfactorily upgrade Claus gas effluent after hydrogenation/hydrolysis. It would also be beneficial if the high water contents in such gases could be reduced so that the production of SO.sub.2 containing waste water streams could be avoided so that water disposal downstream of the clean up stage could be reduced and made less noxious, because of a substantially reduced or eliminated content of SO.sub.2. It would also be beneficial to have a process which can clean up the effluent from a Claus gas - hydrogenation/hydrolysis unit by making elemental sulfur and simultaneously generating an exhaust gas having an H.sub.2 S/SO.sub.2 ratio of approximately 2 to 1, which can be supplied directly to a Claus reactor.